Polyethylene (PE) is consumed in the USA at the rate of over ten million metric tons every year. The semi-crystalline nature of polyethylene makes it a material of choice for many commodity and specialty applications. Crystallinity offers many desirable features to PE such as stiffness, strength, barrier to gas transport, chemical resistance, and dimensional stability. The non-crystalline phase can impart such attributes as toughness and resistance to slow crack growth.
Presently, a variety of PE resins can be used to produce high stiffness pipe used in water, gas, and other fluid transport applications. Polyethylene pipe classified as PE-100, MRS 10, or ASTM D3350 typical cell classification 345566C is especially desirable for use under conditions requiring higher pressure ratings. To obtain a PE-100 classification, PE-100 pipe is required to meet certain standards specifying stiffness, resistance to slow crack growth, resistance to chemical attack, and low-temperature toughness (expressed as rapid crack propagation). Further, such pipe must meet a deformation standard which is determined under pressure at elevated temperatures. Resin which can be employed to produce both small diameter (1 inch to 12 inches in diameter) and large diameter (greater than 12 inches in diameter) PE-100 pipe is described in U.S. Patent Application Pub. No. 2003/0199648 A1 (U.S. patent application Ser. No. 10/373,648), published Oct. 23, 2003, which is incorporated herein by reference in its entirety.
It is also desirable for PE-100 pipe to exhibit toughness. This characteristic can be important in many polyethylene pipe applications, such as, where the pipe is buried underground or where the pipe is used to transport coarse or abrasive slurries. Accordingly, there is a need for a resin and a PE-100 pipe made therefrom which has improved physical properties and impact resistance properties.